add ClusterCost interface (opensource4you#514)

app/src/main/java/org/astraea/app/cost/ClusterCost.java

@@ -0,0 +1,24 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.astraea.app.cost;
+
+/** Return type of cost function, `HasMoveCost`. It returns the score of brokers. */
+public interface ClusterCost {
+
+  /** @return cost of cluster */
+  double value();
+}

app/src/main/java/org/astraea/app/cost/HasClusterCost.java

@@ -0,0 +1,31 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.astraea.app.cost;
+
+import org.astraea.app.admin.ClusterBean;
+import org.astraea.app.admin.ClusterInfo;
+
+public interface HasClusterCost extends CostFunction {
+  /**
+   * score cluster for a particular metrics according to passed beans and cluster information.
+   *
+   * @param clusterInfo cluster information
+   * @param clusterBean cluster metrics
+   * @return the score of cluster.
+   */
+  ClusterCost clusterCost(ClusterInfo clusterInfo, ClusterBean clusterBean);
+}

app/src/main/java/org/astraea/app/cost/ReplicaLeaderCost.java

@@ -28,7 +28,20 @@
 import org.astraea.app.metrics.collector.Fetcher;
 
 /** more replica leaders -> higher cost */
-public class ReplicaLeaderCost implements HasBrokerCost {
+public class ReplicaLeaderCost implements HasBrokerCost, HasClusterCost {
+  static double coefficientVariation(Map<Integer, Double> brokerScore) {
+    var dataRateMean =
+        brokerScore.values().stream().mapToDouble(x -> (double) x).sum() / brokerScore.size();
+    var dataRateSD =
+        Math.sqrt(
+            brokerScore.values().stream()
+                    .mapToDouble(score -> Math.pow((score - dataRateMean), 2))
+                    .sum()
+                / brokerScore.size());
+    var cv = dataRateSD / dataRateMean;
+    if (cv > 1) return 1.0;
+    return cv;
+  }
 
   @Override
   public BrokerCost brokerCost(ClusterInfo clusterInfo, ClusterBean clusterBean) {
@@ -38,6 +51,12 @@ public BrokerCost brokerCost(ClusterInfo clusterInfo, ClusterBean clusterBean) {
     return () -> result;
   }
 
+  @Override
+  public ClusterCost clusterCost(ClusterInfo clusterInfo, ClusterBean clusterBean) {
+    var brokerScore = brokerCost(clusterInfo, clusterBean).value();
+    return () -> coefficientVariation(brokerScore);
+  }
+
   Map<Integer, Integer> leaderCount(ClusterInfo ignored, ClusterBean clusterBean) {
     return clusterBean.all().entrySet().stream()
         .collect(

app/src/test/java/org/astraea/app/cost/ReplicaLeaderCostTest.java

@@ -61,12 +61,14 @@ void testWithMetrics() {
     var broker2 = List.of((HasBeanObject) LeaderCount2);
     var broker3 = List.of((HasBeanObject) LeaderCount3);
     var clusterBean = ClusterBean.of(Map.of(1, broker1, 2, broker2, 3, broker3));
-    var load = costFunction.brokerCost(ClusterInfo.EMPTY, clusterBean);
+    var brokerLoad = costFunction.brokerCost(ClusterInfo.EMPTY, clusterBean);
+    var clusterLoad = costFunction.clusterCost(ClusterInfo.EMPTY, clusterBean);
 
-    Assertions.assertEquals(3, load.value().size());
-    Assertions.assertEquals(3.0, load.value().get(1));
-    Assertions.assertEquals(4.0, load.value().get(2));
-    Assertions.assertEquals(5.0, load.value().get(3));
+    Assertions.assertEquals(3, brokerLoad.value().size());
+    Assertions.assertEquals(3.0, brokerLoad.value().get(1));
+    Assertions.assertEquals(4.0, brokerLoad.value().get(2));
+    Assertions.assertEquals(5.0, brokerLoad.value().get(3));
+    Assertions.assertEquals(0.2041241452319315, clusterLoad.value());
   }
 
   private ServerMetrics.ReplicaManager.Meter mockResult(String name, long count) {